I've done this activity with Years 5 and 6 and it worked well. When certain numbers are put into a calculator they look like letters of the alphabet when viewed upside down. Challenge pupils to generate as many letters as they can using this method. In fact, only eight letters are possible, some uppercase and some lower:
S L B g E I O h
5 7 8 6 3 1 0 4
Once these letters have been identified, the pupils can create a word bank.
They will notice that all the words need spelling backwards. To spell "shell" they need to input the numbers 77345 (boil = 7108; gloss = 55076; hills = 57714; hole = 3704).
As an extension task, ask pupils to find a way to produce a word that ends in "o" - this will require a decimal point (eg, hello needs the input .7734).
The class should then be ready to create calculations of their own that result in a given word (eg, when inverted 9619 x 6 = 57714 gives "hills").
The sums can be varied - highflyers can be given a checklist of maths concepts, such as square-roots and using brackets, and be instructed to include them in their calculations. Starting with an answer and creating a sum for it also provides a perfect opportunity to explore inverse operations. Once these skills are mastered, pupils can devise their own "calculator crossword". Each clue needs to have a sum to work out, included in a sentence. For example: Across: 1. 4 x 14277 for what water does at 100 degrees C. (Answer: boils.)
The completed crosswords can be photocopied and distributed to the pupils during the following lesson.
Art co-ordinator, Colmore Junior School,Kings Heath, Birmingham